Cnt-infused fiber and method therefor

ABSTRACT

A carbon nanotube-infused fiber and a method for its production are disclosed. Nanotubes are synthesized directly on a parent fiber by first applying a catalyst to the fiber. The properties of the carbon nanotube-infused fiber will be a combination of those of the parent fiber as well as those of the infused carbon nanotubes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 11/619,327, filed on Jan. 3, 2007.

FIELD OF THE INVENTION

The present invention relates to carbon nanotubes and fibers.

BACKGROUND OF THE INVENTION

Fibers are used for many different applications in a wide variety of industries, such as the commercial aviation, recreation, industrial and transportation industries. Commonly-used fibers for these and other applications include cellulosic fiber (e.g., viscose rayon, cotton, etc.), glass fiber, carbon fiber, and aramid fiber, to name just a few.

In many fiber-containing products, the fibers are present in the form of a composite material (e.g., fiberglass, etc.). A composite material is a heterogeneous combination of two or more constituents that differ in form or composition on a macroscopic scale. While the composite material exhibits characteristics that neither constituent alone possesses, the constituents retain their unique physical and chemical identities within the composite.

Two key constituents of a composite include a reinforcing agent and a resin matrix. In a fiber-based composite, the fibers are the reinforcing agent. The resin matrix keeps the fibers in a desired location and orientation and also serves as a load-transfer medium between fibers within the composite.

Fibers are characterized by certain properties, such as mechanical strength, density, electrical resistivity, thermal conductivity, etc. The fibers “lend” their characteristic properties, in particular their strength-related properties, to the composite. Fibers therefore play an important role in determining a composite's suitability for a given application.

To realize the benefit of fiber properties in a composite, there must be a good interface between the fibers and the matrix. This is achieved through the use of a surface coating, typically referred to as “sizing.” The sizing provides an all important physico-chemical link between fiber and the resin matrix and thus has a significant impact on the mechanical and chemical properties of the composite. The sizing is applied to fibers during their manufacture.

Substantially all conventional sizing has lower interfacial strength than the fibers to which it's applied. As a consequence, the strength of the sizing and its ability to withstand interfacial stress ultimately determines the strength of the overall composite. In other words, using conventional sizing, the resulting composite cannot have a strength that is equal to or greater than that of the fiber.

SUMMARY OF THE INVENTION

The illustrative embodiment of the present invention is a carbon nanotube-infused (“CNT-infused”) fiber.

In CNT-infused fiber disclosed herein, the carbon nanotubes are “infused” to the parent fiber. As used herein, the term “infused” means physically or chemically bonded and “infusion” means the process of physically or chemically bonding. The physical bond between the carbon nanotubes and parent fiber is believed to be due, at least in part, to van der Waals forces. The chemical bond between the carbon nanotubes and the parent fiber is believed to be a covalent bond.

Regardless of its true nature, the bond that is formed between the carbon nanotubes and the parent fiber is quite robust and is responsible for CNT-infused fiber being able to exhibit or express carbon nanotube properties or characteristics. This is in stark contrast to some prior-art processes, wherein nanotubes are suspended/dispersed in a solvent solution and applied, by hand, to fiber. Because of the strong van der Waals attraction between the already-formed carbon nanotubes, it is extremely difficult to separate them to apply them directly to the fiber. As a consequence, the lumped nanotubes weakly adhere to the fiber and their characteristic nanotube properties are weakly expressed, if at all.

The infused carbon nanotubes disclosed herein effectively function as a replacement for conventional “sizing.” It has been found that infused carbon nanotubes are far more robust molecularly and from a physical properties perspective than conventional sizing materials. Furthermore, the infused carbon nanotubes improve the fiber-to-matrix interface in composite materials and, more generally, improve fiber-to-fiber interfaces.

The CNT-infused fiber disclosed herein is itself similar to a composite material in the sense that its properties will be a combination of those of the parent fiber as well as those of the infused carbon nanotubes. Consequently, embodiments of the present invention provide a way to impart desired properties to a fiber that otherwise lacks such properties or possesses them in insufficient measure. Fibers can therefore be tailored or engineered to meet the requirements of a specific application. In this fashion, the utility and value of virtually any type of fiber can be improved.

In accordance with the illustrative embodiment of a CNT-infused fiber-forming process, nanotubes are synthesized in place on the parent fiber itself. It is important that the carbon nanotubes are synthesized on the parent fiber. If not, the carbon nanotubes will become highly entangled and infusion does not occur. As seen from the prior art, non-infused carbon nanotubes impart little if any of their characteristic properties.

The parent fiber can be any of a variety of different types of fibers, including, without limitation: carbon fiber, graphite fiber, metallic fiber (e.g., steel, aluminum, etc.), ceramic fiber, metallic-ceramic fiber, glass fiber, cellulosic fiber, aramid fiber.

In the illustrative embodiment, nanotubes are synthesized on the parent fiber by applying or infusing a nanotube-forming catalyst, such as iron, nickel, cobalt, or a combination thereof, to the fiber.

In some embodiments, operations of the CNT-infusion process include:

-   -   Removing sizing from the parent fiber;     -   Applying nanotube-forming catalyst to the parent fiber;     -   Heating the fiber to nanotube-synthesis temperature; and     -   Spraying carbon plasma onto the catalyst-laden parent fiber.

In some embodiments, the infused carbon nanotubes are single-wall nanotubes. In some other embodiments, the infused carbon nanotubes are multi-wall nanotubes. In some further embodiments, the infused carbon nanotubes are a combination of single-wall and multi-wall nanotubes. There are some differences in the characteristic properties of single-wall and multi-wall nanotubes that, for some end uses of the fiber, dictate the synthesis of one or the other type of nanotube. For example, single-walled nanotubes can be excellent conductors of electricity while multi-walled nanotubes are not.

Methods and techniques for forming carbon nanotubes, as disclosed in co-pending U.S. patent application Ser. No. 10/455,767 (now U.S. Pat. No. 7,261,779) and which is incorporated herein by reference, can be adapted for use with the process described herein. In the illustrative embodiment, acetylene gas is ionized to create a jet of cold carbon plasma. The plasma is directed toward the catalyst-bearing parent fiber.

As previously indicated, carbon nanotubes lend their characteristic properties (e.g., exceptional mechanical strength, low to moderate electrical resistivity, high thermal conductivity, etc.) to the CNT-infused fiber. The extent to which the resulting CNT-infused fiber expresses these characteristics is a function of the extent and density of coverage of the parent fiber by the carbon nanotubes.

In a variation of the illustrative embodiment, CNT infusion is used to provide an improved filament winding process: In this variation, carbon nanotubes are formed on fibers (e.g., graphite tow, glass roving, etc.), as described above, and are then passed through a resin bath to produce resin-impregnated, CNT-infused fiber. After resin impregnation, the fiber is positioned on the surface of a rotating mandrel by a delivery head. The fiber then winds onto the mandrel in a precise geometric pattern in known fashion.

The filament winding process described above provides pipes, tubes, or other forms as are characteristically produced via a male mold. But the forms made from the filament winding process disclosed herein differ from those produced via conventional filament winding processes. Specifically, in the process disclosed herein, the forms are made from composite materials that include CNT-infused fibers. Such forms will therefore benefit from enhanced strength, etc., as provided by the CNT-infused fibers.

Any of a variety of different parent fibers can be used to form CNT-infused fiber.

Of late, there has been a demand for carbon fiber forms that are compatible with a broad range of resins and processes, and the sizing material is an important determinant of this compatibility. For example, sizing is critically important for providing an even distribution of chopped carbon fiber in sheet molding compounds (“SMCs”), such as are used in some automotive body panels.

Notwithstanding this demand for carbon fiber and its potentially broad applicability, carbon fiber has historically been sized for compatibility with only epoxy resin. CNT-infused carbon fiber, as produced according to the method disclosed herein, addresses this problem by providing a fiber that is sized with infused nanotubes, which provides the desired broad applicability with a variety of resins and processes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a method for producing CNT-infused fiber in accordance with the illustrative embodiment of the present invention.

FIG. 2 depicts a system for implementing the illustrative method for producing CNT-infused fiber.

FIG. 3 depicts a system for filament winding in accordance with a variation of the illustrative embodiment.

DETAILED DESCRIPTION

The following terms are defined for use in this Specification, including the appended claims:

-   -   Carding—The process by which the fibers are opened out into an         even film.     -   Carded Fibers—Fibers that have been carded which opens them up.     -   Cloth—A reinforcement material made by weaving strands of fiber         yarns.     -   Continuous Filament Strand—A fiber bundle composed of many         filaments.     -   Also, when referring to gun roving; a collection of string-like         fiber or yarn, which is fed through a chopper gun in a spray-up         process.     -   Continuous Strand Roving—A bundle of filaments which are fed         through a chopper gun in a spray-up process.     -   Fabric—A planar textile structure produced by interlacing yarns,         fibers, or filaments.     -   Fiber—A unit of matter, either natural, or manufactured, which         forms the basic element of fabrics and other textile structures.     -   Fiber orientation—Fiber alignment in a non-woven or a mat         laminate where the majority of fibers are in the same direction,         resulting in a higher strength in that direction.     -   Fiber Pattern—Visible fibers on the surface of laminates or         moldings; the thread size and weave of glass cloth.     -   Filament—A single fiber of an indefinite or extreme length,         either natural (e.g., silk, etc.) or manufactured. Typically         microns in diameter, manufactured fibers are extruded into         filaments that are converted into filament yarn, staple, or tow.     -   Filament Winding—A process which involves winding a         resin-saturated strand of glass filament around a rotating         mandrel.     -   Filament Yarn—A yarn composed of continuous filaments assembled         with, or without twist.     -   Infuse—To form a chemical bond.     -   Male Mold—A convex mold where the concave surface of the part is         precisely defined by the mold surface.     -   Matrix—The liquid component of a composite or laminate.     -   Mandrel—The core around which paper-, fabric-, or         resin-impregnated fiber is wound to form pipes, tubes, or         vessels; in extrusion, the central finger of a pipe or tubing         die.     -   Pultrusion—Reversed “extrusion” of resin-impregnated roving in         the manufacture of rods, tubes and structural shapes of a         permanent cross-section. The roving, after passing through the         resin dip tank, is drawn through a die to form the desired         cross-section.     -   Resin—A liquid polymer that, when catalyzed, cures to a solid         state.     -   Roving—The soft strand of carded fiber that has been twisted,         attenuated, and freed of foreign matter preparatory to spinning.     -   Sizing—A surface treatment that is applied to filaments         immediately after their formation for the purpose of promoting         good adhesion between those filaments and the matrix, to the         extent the filaments are to be used as the reinforcing agent in         a composite material.     -   Spray-up—The process of spraying fibers, resin and catalyst         simultaneously into a mold using a chopper gun.     -   Strands—A primary bundle of continuous filaments (or slivers)         combined in a single compact unit without twist. These filaments         (usually 51, 102 or 204) are gathered together in the forming         operations.     -   Tape—A narrow-width reinforcing fabric or mat.     -   Tow—A loose strand of filaments without twist.     -   Twist—A term that applies to the number of turns and the         direction that two yarns are turned during the manufacturing         process.     -   Woven Roving Fabric—Heavy fabrics woven from continuous filament         in roving form. Usually in weights between 18-30 oz. per square         yard.     -   Yarn—A generic term for a continuous strand of textile fibers,         filaments, or material in a form suitable for knitting, weaving,         braiding, or otherwise intertwining to form a textile fabric.

As the definitions that are provided above indicate, terms such as “fiber,” “filament,” “yarn,” etc., have distinct meanings. But for the purposes of the specification and the appended claims, and unless otherwise indicated, the term “fiber” is used in this specification as a generic term to refer to filament, yarn, tow, roving, fabric, etc., as well as fiber itself. The phrase “CNT-infused fiber” is therefore understood to encompass “CNT-infused fiber,” “CNT-infused filament,” “CNT-infused tow,” CNT-infused roving,” etc.

FIG. 1 depicts a flow diagram of process 100 for producing. CNT-infused fiber in accordance with the illustrative embodiment of the present invention.

Process 100 includes the operations of:

-   -   102: Applying nanotube-forming catalyst to the parent fiber.     -   104: Heating the parent fiber to a temperature that is         sufficient for carbon nanotube synthesis.     -   106: Spraying carbon plasma onto the catalyst-laden parent         fiber.

To infuse carbon nanotubes into a parent fiber, the carbon nanotubes are synthesized directly on the parent fiber. In the illustrative embodiment, this is accomplished by disposing nanotube-forming catalyst on the parent fiber, as per operation 102. Suitable catalysts for carbon nanotube formation include, without limitation, transition metal catalysts (e.g., iron, nickel, cobalt, combinations thereof, etc.).

As described further in conjunction with FIG. 2, the catalyst is prepared as a liquid solution that contains nano-sized particles of catalyst. The diameters of the synthesized nanotubes are related to the size of the metal particles.

In the illustrative embodiment, carbon nanotube synthesis is based on a plasma-enhanced chemical vapor deposition process and occurs at elevated temperatures. The temperature is a function of catalyst, but will typically be in a range of about 500 to 1000° C. Accordingly, operation 104 requires heating the parent fiber to a temperature in the aforementioned range to support carbon nanotube synthesis.

In operation 106, carbon plasma is sprayed onto the catalyst-laden parent fiber. The plasma can be generated, for example, by passing a carbon containing gas (e.g., acetylene, ethylene, ethanol, etc.) through an electric field that is capable of ionizing the gas.

Nanotubes grow at the sites of the metal catalyst. The presence of the strong plasma-creating electric field can affect nanotube growth. That is, the growth tends to follow the direction of the electric field. By properly adjusting the geometry of the plasma spray and electric field, vertically-aligned carbon nanotubes (i.e., perpendicular to the fiber) are synthesized. Under certain conditions, even in the absence of a plasma, closely-spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest.

FIG. 2 depicts system 200 for producing CNT-infused fiber in accordance with the illustrative embodiment of the present invention. System 200 includes fiber payout and tensioner station 202, fiber spreader station 208, sizing removal station 210, CNT-infusion station 212, fiber bundler station 222, and fiber uptake bobbin 224, interrelated as shown.

Payout and tension station 202 includes payout bobbin 204 and tensioner 206. The payout bobbin delivers fiber 201 to the process; the fiber is tensioned via tensioner 206.

Fiber 201 is delivered to fiber spreader station 208. The fiber spreader separates the individual elements of the fiber. Various techniques and apparatuses can be used to spread fiber, such as pulling the fiber over and under flat, uniform-diameter bars, or over and under variable-diameter bars, or over bars with radially-expanding grooves and a kneading roller, over a vibratory bar, etc. Spreading the fiber enhances the effectiveness of downstream operations, such as catalyst application and plasma application, by exposing more fiber surface area.

Payout and tension station 202 and fiber spreader station 208 are routinely used in the fiber industry; those skilled in the art will be familiar with their design and use.

Fiber 201 then travels to sizing removal station 210. At this station, any “sizing” that is on fiber 201 is removed. Typically, removal is accomplished by burning the sizing off of the fiber.

Any of a variety of heating means can be used for this purpose, including, without limitation, an infrared heater, a muffle furnace, etc. Generally, non-contact heating methods are preferred. In some alternative embodiments, sizing removal is accomplished chemically.

The temperature and time required for burning off the sizing vary as a function of (1) the sizing material (e.g., silane, etc.); and (2) the identity of parent fiber 201 (e.g., glass, cellulosic, carbon, etc.). Typically, the burn-off temperature is a minimum of about 650° C. At this temperature, it can take as long as 15 minutes to ensure a complete burn off of the sizing. Increasing the temperature above a minimum burn temperature should reduce burn-off time. Thermogravimetric analysis can be used to determine minimum burn-off temperature for sizing.

In any case, sizing removal is the slow step in the overall CNT-infusion process. For this reason, in some embodiments, a sizing removal station is not included in the CNT-infusion process proper; rather, removal is performed separately (e.g., in parallel, etc.). In this way, an inventory of sizing-free fiber can be accumulated and spooled for use in a CNT-infused fiber production line that does not include a fiber removal station). In such embodiments, sizing-free fiber is spooled in payout and tension station 202. This production line can be operated at higher speed than one that includes sizing removal.

Sizing-free fiber 205 is delivered to CNT-infusion station 212, which is the “heart” of the process and system depicted in FIG. 2. Station 212 includes catalyst application station 214, fiber pre-heater station 216, plasma spray station 218, and fiber heaters 220.

As depicted in FIG. 2, sizing-free fiber 205 proceeds first to catalyst application station 214. In some embodiments, fiber 205 is cooled prior to catalyst application.

In some embodiments, the nanotube-forming catalyst is a liquid solution of nanometer-sized particles (e.g., 10 nanometers in diameter, etc.) of a transition metal. Typical transition metals for use in synthesizing nanotubes include, without limitation, iron, iron oxide, cobalt, nickel, or combinations thereof. These transition metal catalysts are readily commercially available from a variety of suppliers, including Ferrotech of Nashua, N.H. The liquid is a solvent such as toluene, etc.

In the illustrative embodiment, the catalyst solution is sprayed, such as by air sprayer 214, onto fiber 205. In some other embodiments, the transition metal catalyst is deposited on the parent fiber using evaporation techniques, electrolytic deposition techniques, suspension dipping techniques and other methods known to those skilled in the art. In some further embodiments, the transition metal catalyst is added to the plasma feedstock gas as a metal organic, metal salt or other composition promoting gas phase transport. The catalyst can be applied at room temperature in the ambient environment (neither vacuum nor an inert atmosphere is required).

Catalyst-laden fiber 207 is then heated at fiber preheater station 216. For the infusion process, the fiber should be heated until it softens. Generally, a good estimate of the softening temperature for any particular fiber is readily obtained from reference sources, as is known to those skilled in the art. To the extent that this temperature is not a priori known for a particular fiber, it can be readily determined by experimentation. The fiber is typically heated to a temperature that is in the range of about 500 to 1000° C. Any of a variety of heating elements can be used as the fiber preheater, such as, without limitation, infrared heaters, a muffle furnace, and the like.

After preheating, fiber 207 is finally advanced to plasma spray station having spray nozzles 218. A carbon plasma is generated, for example, by passing a carbon containing gas (e.g., acetylene, ethylene, ethanol, etc.) through an electric field that is capable of ionizing the gas. This cold carbon plasma is directed, via spray nozzles 218, to fiber 207. The fiber is disposed within about 1 centimeter of the spray nozzles to receive the plasma. In some embodiments, heaters 220 are disposed above fiber 207 at the plasma sprayers to maintain the elevated temperature of the fiber.

After CNT-infusion, CNT-infused fiber 209 is re-bundled at fiber bundler 222. This operation recombines the individual strands of the fiber, effectively reversing the spreading operation that was conducted at station 208.

The bundled, CNT-infused fiber 209 is wound about uptake fiber bobbin 224 for storage. CNT-infused fiber 209 is then ready for use in any of a variety of applications, including, without limitation, for use as the reinforcing material in composite materials.

It is noteworthy that some of the operations described above should be conducted under inert atmosphere or vacuum, such that environmental isolation is required. For example, if sizing is being burned off of the fiber, the fiber must be environmentally isolated to contain off-gassing and prevent oxidation. Furthermore, the infusion process should be conducted under an inert atmosphere (e.g., nitrogen, argon, etc.) to prevent oxidation of the carbon. For convenience, in some embodiments of system 200, environmental isolation is provided for all operations, with the exception of fiber payout and tensioning (at the beginning of the production line) and fiber uptake (at the end of the production line).

FIG. 3 depicts a further embodiment of the invention wherein CNT-infused fiber is created as a sub-operation of a filament winding process being conducted via filament winding system 300.

System 300 comprises fiber creel 302, carbon nanotube infusion section 226, resin bath 328, and filament winding mandrel 332, interrelated as shown. The various elements of system 300, with the exception of carbon nanotube infusion section 226, are present in conventional filament winding processes. Again, the “heart” of the process and system depicted in FIG. 3 is the carbon nanotube infusion section 226, which includes fiber spreader station 208, (optional) sizing-removal station 210, and CNT-infusion station 212.

Fiber creel 302 includes plural spools 204 of parent fiber 201A through 201H. The untwisted group of fibers 201A through 201H is referred to collectively as “tow 303.” Note that the term “tow” generally refers to a group of graphite fibers and the term “roving” usually refers to glass fibers. Here, the term “tow” is meant to refer, generically, to any type of fiber.

In the illustrative embodiment, creel 302 holds spools 204 in a horizontal orientation. The fiber from each spool 206 moves through small, appropriately situated rollers/tensioners 206 that change the direction of the fibers as they move out of creel 302 and toward carbon nanotube infusion section 226.

It is understood that in some alternative embodiments, the spooled fiber that is used in system 300 is CNT-infused fiber (i.e., produced via system 200). In such embodiments, system 300 is operated without nanotube infusion section 226.

In carbon nanotube infusion section 226, tow 303 is spread, sizing is removed, nanotube-forming catalyst is applied, the tow is heated, and carbon plasma is sprayed on the fiber, as described in conjunction with FIG. 2.

After passing through nanotube infusion section 226, CNT-infused tow 307 is delivered to resin bath 328. The resin bath contains resin for the production of a composite material comprising the CNT-infused fiber and the resin. Some important commercially-available resin-matrix families include general purpose polyester (e.g., orthophthalic polyesters, etc.), improved polyester (e.g., isophthalic polyesters, etc.), epoxy, and vinyl ester.

Resin bath can be implemented in a variety of ways, two of which are described below. In the illustrative embodiment, resin bath 328 is implemented as a doctor blade roller bath wherein a polished rotating cylinder (e.g., cylinder 330) that is disposed in the bath picks up resin as it turns. The doctor bar (not depicted in FIG. 3) presses against the cylinder to obtain a precise resin film thickness on cylinder 330 and pushes excess resin back into the bath. As fiber tow 307 is pulled over the top of cylinder 330, it contacts the resin film and wets out. In some other embodiments, resin bath 328 is realized as an immersion bath wherein fiber tow 307 is simply submerged into resin and then pulled through a set of wipers or roller that remove excess resin.

After leaving resin bath 328, resin-wetted, CNT-infused fiber tows 309 is passed through various rings, eyelets and, typically, a multi-pin “comb” (not depicted) that is disposed behind a delivery head (not depicted). The comb keeps the fiber tows 309 separate until they are brought together in a single combined band on rotating mandrel 332.

Example

A CNT-infused carbon fiber was formed in accordance with the illustrative embodiment. A current was passed through carbon fiber (the parent fiber) to heat it to approximately 800° C. to remove epoxy sizing material. The fiber was then cooled to room temperature and left clamped between electrodes. A ferro-fluid catalyst was applied to the fiber using an aerosol spray technique. The fiber was allowed to dry and the chamber was closed, evacuated and filled with argon. A current was passed through the carbon fiber again to heat it to approximately 800° C. for carbon nanotube synthesis. A carbon plasma was generated from acetylene precursor using 13.56 MHz microwave energy using an atmospheric pressure plasma jet. The carrier gas in the plasma jet was helium at 20 standard liters per minute (slm) and the argon was provided at 1.2 slm. The plasma jet was fixtured to a robotic motion control system allowing the plasma jet to move over the length of the fiber at a speed between 6 and 12 inches per minute. The CNT-infused fiber was then cooled to room temperature and removed from the chamber. Scanning Electron Microscopy showed carbon nanotube formation on the surface of the parent carbon fiber.

It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc.

Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents. 

What is claimed is the following:
 1. A method of infusing carbon nanotubes on a parent fiber, the method comprising: applying a carbon nanotube-forming catalyst on a surface of a parent fiber comprising a plurality of filaments, thereby forming a catalyst-laden fiber; pre-heating the catalyst-laden fiber to a temperature between about 500° C. and about 1000° C. prior to synthesizing carbon nanotubes on the catalyst-laden fiber; and while maintaining the catalyst-laden fiber at a temperature ranging between about 500° C. and about 1000° C., synthesizing carbon nanotubes on the catalyst-laden fiber while the catalyst-laden fiber is being transported, thereby forming a carbon nanotube-infused fiber.
 2. The method of claim 1, further comprising: removing a sizing material from the parent fiber prior to applying a carbon nanotube-forming catalyst thereto.
 3. The method of claim 1, further comprising: applying a resin to the carbon nanotube-infused fiber.
 4. The method of claim 3, further comprising: winding the carbon nanotube-infused fiber about a mandrel after applying the resin.
 5. The method of claim 1, further comprising: spreading the filaments of the parent fiber.
 6. The method of claim 5, re-bundling the filaments of the spread parent fiber.
 7. The method of claim 1, wherein the carbon nanotube-forming catalyst comprises a transition metal.
 8. The method of claim 1, wherein the carbon nanotubes are synthesized in a carbon plasma at atmospheric pressure.
 9. The method of claim 1, wherein the parent fiber comprises a fiber tow.
 10. A system for producing carbon nanotube-infused fibers, the system comprising: a carbon nanotube infusion station comprising, in series, a catalyst application station, a heater, and a plasma spray station; and a transport mechanism operable for conveying a fiber comprising a plurality of filaments through the carbon nanotube infusion station.
 11. The system of claim 10, further comprising: a station for spreading the fiber before the carbon nanotube infusion station.
 12. The system of claim 11, further comprising: a station for re-bundling the spread fiber after the carbon nanotube infusion station.
 13. The system of claim 10, further comprising: a station for removing a sizing material from the fiber before the carbon nanotube infusion station.
 14. The system of claim 13, wherein the station for removing a sizing material comprises a heater.
 15. The system of claim 10, wherein the plasma spray station comprises a plurality of carbon plasma sprayers.
 16. The system of claim 15, wherein the plasma spray station further comprises heaters at the carbon plasma sprayers.
 17. The system of claim 10, further comprising: a resin bath after the carbon nanotube infusion station.
 18. A composition comprising a carbon nanotube-infused fiber, wherein the carbon nanotube-infused fiber comprises: a parent fiber comprising a plurality of filaments; and a plurality of carbon nanotubes covalently bonded to the parent fiber.
 19. The composition of claim 18, further comprising a resin.
 20. The composition of claim 18, wherein the parent fiber comprises a fiber tow.
 21. The composition of claim 18, wherein the carbon nanotube-infused fiber is prepared by a process comprising: applying a carbon nanotube-forming catalyst on a surface of a parent fiber comprising a plurality of filaments, thereby forming a catalyst-laden fiber; pre-heating the catalyst-laden fiber to a temperature between about 500° C. and about 1000° C. prior to synthesizing carbon nanotubes on the catalyst-laden fiber; and while maintaining the catalyst-laden fiber at a temperature ranging between about 500° C. and about 1000° C., synthesizing carbon nanotubes on the catalyst-laden fiber while the catalyst-laden fiber is being transported.
 22. The composition of claim 18, wherein the parent fiber comprises a carbon fiber lacking a sizing material.
 23. The composition of claim 18, wherein the electrical resistivity of the carbon nanotube-infused fiber is lower than the electrical resistivity of the parent fiber. 